Reactive hot melts are one-component, 100% solid, solvent-free urethane prepolymers. Unlike conventional hot melts that can be repeatedly heated from its solid state and flowed to a liquid form, the reactive hot melt behaves as a thermoset and goes through an irreversible chemical reaction once dispensed in the presence of ambient moisture.
The reactive hot melts are isocyanate terminated prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane polymer which offers performance superior to that obtained with conventional hot melt.
These reactive hot melts will bond to a variety of substrates including plastics, woods, fabrics and some metals making them ideal candidates for bonding dissimilar substrates. Moreover, they are flexible and durable by nature, so they may be used in extreme temperature ranges of -30.degree. to +150.degree. C. while providing excellent moisture and chemical resistance.
While recent progress has been made to improve the green strength and other properties of these adhesives, there remains a need to improve their curing speed. Catalysts have often been employed for this purpose; however, not without sacrifice of other properties. Thus, organo tin and bismuth catalysts such as dibutyl tin dilaurate are known to boost the curing speed of reactive polyurethane hot melts but the use thereof is accompanied by a reduction in thermal stability. Amine catalysts such as ethylene diamine have provided better thermal stability but are not as effective catalysts.
Therefore, there remains a need for a catalyst which will improve the curing speed of reactive hot melt adhesives without significantly reducing the thermal stability.